The freezing-thawing resistance of a cement composition is, along with a shrinkage reducing effect, one of the important factors in the durability of a cement composition. Having freezing-thawing resistance is, especially in cold regions, an essential requirement for cement compositions. As a technique for improving freezing-thawing resistance, a frost damage resistant cement admixture material characterized by containing an AE agent having a high-range water reducing agent and a natural resinate as main components has been reported (Japanese Patent Application Laid-Open No. 2000-95551 (Patent document 1)). However, the effect provided by this admixture on freezing-thawing resistance is insufficient. Moreover, if a shrinkage reducing agent expected to give a shrinkage reducing effect is added to a cement composition additionally with this admixture, it has caused the trouble that the freezing-thawing resistance of the cement composition is dramatically harmed. Conventionally, the only way to ensure freezing-thawing resistance has been to increase the strength of the cement composition or carry out measures during the curing process after the cement composition has been produced, and even the resultant effects were insufficient. Therefore, there is an expectation of developing a new durability improving agent which simply improves freezing-thawing resistance.
The entrainment of fine air bubbles in a cement hydrate is very effective in improving freezing-thawing resistance. Consequently, it is common to add an air entraining component to a cement composition. However, for the following reasons, this cannot always be a means that prevents frost damage.
A first reason that frost damage cannot be prevented by the addition of an air entraining component is that, although defoaming occurs while the cement hydrate is hardening, the air entraining component is added before the hardening of the cement hydrate, and consequently, after hardening, in some cases the air bubbles cannot be sufficiently entrained after the hardening.
A second reason that frost damage cannot be prevented by the addition of an air entraining component is that since ice nuclei are necessary to form ice in the cement hydrate, the contribution of the air content in the cement hydrate in suppressing frost damage may possibly not be very much. The formation of ice may also be said to be a random, haphazard phenomenon.
A third reason that frost damage cannot be prevented by the addition of an air entraining component is that the incidence of frost damage differs greatly depending on the materials used in the cement composition. For example, when using an aggregate having a large percentage of total moisture content, an aggregate having a low strength, or a cement with slow strength development as the cement hydrate materials, the cement composition is susceptible to frost damage due to the large amount of frozen water in the cement hydrate and the fact that the composition lacks the strength to withstand freeze expansion.
A fourth reason that frost damage cannot be prevented by the addition of an air entraining component is that the concentration of specific components included in the cement hydrate increases when the cement hydrate freezes, which has a collateral effect of accelerating deterioration. Reported examples of such components include chlorine ions and hydrogen ions. An increased concentration of chlorine ions induces supercooling due to the increased mole concentration, causing ice lenses to be produced in the cement hydrate. An increased concentration of hydrogen ions is a factor in alkali-aggregate reactions.
A fifth reason that frost damage cannot be prevented by the addition of an air entraining component is that the pore structure in the cement hydrate affects frost damage. The freezing of a cement composition proceeds in a manner that is dependent on pore diameter. Specifically, the moisture in coarser pores is less susceptible to supercooling, so that freezing occurs at a higher temperature. Even when there is high continuity of the pores in the cement hydrate, the amount of frozen water increases due to the continuous ice trapping non-frozen water, whereby frost damage is manifested. Thus, since there are various causes of frost damage, an improvement will not necessarily be achieved merely by introducing air into the cement composition.
In addition, even when air is entrained by adding an air entraining agent to the cement composition, if a shrinkage reducing agent that is not attached to the cement particles is also used, the shrinkage reducing agent causes the diameter of the entrained air bubbles to expand. As a result, the diameter of air bubble and the air-void spacing factor increase, so that the freezing-thawing resistance dramatically deteriorates. Generally, if the air-void spacing factor is 250 μm or less, freezing-thawing resistance is said to be excellent. However, it has been reported that even when the air-void spacing factor is 250 μm or less, if a shrinkage reducing agent that is not attached to the cement particles is included in the moisture in the pores in the cement composition, the freezing-thawing resistance of the cement composition is dramatically harmed (Concrete Annual Journal, July 2007, Vol. 30, 1188 (refer to Non-Patent document 1)).
On the other hand, typically a shrinkage reducing agent for a cement composition (hereinafter, sometimes referred to as “shrinkage reducing agent”) is used for the purpose of reducing the amount of drying shrinkage, which has an adverse effect on the durability of the obtained cement composition. Shrinkage reducing agents may be compounds that maintain water solubility (Japanese Patent Application Laid-Open No. 2001-163653 (Patent document 2)), or compounds that are water-insoluble (for example, refer to Japanese Patent Application Laid-Open No. Hei 2-124750 (Patent document 3)).
However, although the shrinkage suppression amount of the cement composition increases with an increase in the added amount for either of these shrinkage reducing agents, there has been the problem that the freezing-thawing resistance of the cement composition is caused to dramatically deteriorate. The only way to avoid this problem and maintain freezing-thawing resistance is to decrease the added amount of the shrinkage reducing agent, which makes it impossible to ensure the desired effect of shrinkage reducing in the cement composition. It has been reported that frost damage resistance (freezing-thawing resistance) of a cement composition added a shrinkage reducing agent dramatically deteriorates even if the added amount of the shrinkage reducing agent is small (documentation from a symposium held by the Architectural Institute of Japan concerning the standardization of concrete materials, Proceedings of Cement/Concrete Admixture Materials and the Current State of the Techniques Relating to the Standardization of the Cement/Concrete Admixture Materials, and a Collection of Papers Thereof, September 2006, p. 82 (refer to Non-Patent document 2)). Therefore, the limit of the shrinkage suppression amount of a conventional shrinkage reducing agent has been thought to be typically a shrinkage suppression ratio of about 15 to 20%.
Thus, in cold regions, it is especially difficult to apply a shrinkage reducing agent as a shrinkage suppression measure for a cement composition. The only way to maintain freezing-thawing resistance is to decrease the added ratio of the shrinkage reducing agent. As a result, a cement composition is unable to achieve a sufficient shrinkage reducing effect, which causes cracking under a constraint condition of a structure and the like. Further, it is difficult to ensure the durability of such a cement composition. In addition, it is known that cement compositions having a crack formed therein suffer from accelerated frost damage deterioration.
Meanwhile, there is a technique for preventing the occurrence of algae and seaweed and for improving seawater resistance, sulfate resistance, acid resistance and the like by adding an aliphatic-based hydrocarbon that is a solid at ordinary temperature to a cement composition (refer to Japanese Patent Application Laid-Open No. Hei 8-26798 (Patent document 4)). However, this technique suffers from the problem that the fluidity and the strength of the cement composition dramatically deteriorate along with an increase in the mixed amount.
Almost no example has been reported with respect to using oils and fats, which are hydrophobic compounds, as a cement composition additive. The reason for this is mainly because oils and fats degrade in an alkali environment, and thus have an adverse effect in the cement composition, such as preventing complete hardening. Another reason is that vegetable oils and animal oils are recognized as substances which corrode concrete. Further, supposing that an oil or fat is added to a cement composition, since the oil or fat has a smaller density than the materials normally used in the cement composition, the oil or fat would be expected to precipitate on the surface of the cement composition due to precipitation separation.